WO2021019153A1

WO2021019153A1 - Method for liquefying natural gas with improved circulation of a mixed refrigerant stream

Info

Publication number: WO2021019153A1
Application number: PCT/FR2020/051319
Authority: WO
Inventors: Natacha Haik-Beraud; Jean-Marc Peyron; Sophie LAZZARINI; Thomas Morel
Original assignee: L'air Liquide Societe Anonyme Pour L'etude Et L'exploitation Des Procedes Georges Claude
Priority date: 2019-08-01
Filing date: 2020-07-21
Publication date: 2021-02-04
Also published as: AU2020323727A1; FR3099557A1; FR3099557B1; US20220275997A1

Abstract

The invention concerns a method for liquefying a hydrocarbon stream (102) such as natural gas using a heat exchanger (E2) with plates (2) having a length measured in the longitudinal direction (z), the plates (2) defining between them at least a first series of passages (10) for the flow of at least a part of a two-phase refrigerant stream (203) vaporising against the hydrocarbon stream (102), the method comprising in particular the steps of: introducing a refrigerant stream (202) into the exchanger (E2), which flows in a downward direction, outputting the refrigerant stream (201 from the exchanger (E2) and expanding so as to produce a two-phase refrigerant stream (203), reintroducing at least a part of the two-phase refrigerant stream (203) into the exchanger (E2), which flows in the passages (10) in an upward direction, at least partially vaporising said at least one part of the two-phase refrigerant stream (203) against the hydrocarbon stream (102) so as to obtain an at least partially liquefied hydrocarbon stream (220). According to the invention, at least one passage (10) comprises a heat exchange structure (S) comprising a plurality of series of walls for guiding fluid succeeding one another in the longitudinal direction (z) and having leading edges which extend orthogonally to the longitudinal direction (z) so as to face, wholly or partially, the two-phase refrigerant stream (203), and having a cross-sectional area (A) of leading edges, measured orthogonally to the longitudinal direction (z) and expressed per metre of heat exchanger length, decreasing in the longitudinal direction (z).

Description

Natural gas liquefaction process with improved circulation of a mixed refrigerant stream

The present invention relates to a process for liquefying a stream

of hydrocarbons, such as natural gas, said process employing a mixed two-phase refrigerant stream which vaporizes against the stream of hydrocarbons to be liquefied in a plate-and-fin type heat exchanger.

It is desirable to liquefy natural gas for a number of reasons. As an example, natural gas can be stored and transported over long distances more easily in the liquid state than in the gaseous state, because it occupies a smaller volume for a given mass and no need to be stored at high pressure.

There are several methods of liquefying a stream of natural gas to obtain liquefied natural gas (LNG). Typically, a refrigerant stream, generally a mixture of several constituents, such as a mixture containing hydrocarbons, is compressed by a compressor and then introduced into an exchanger where it is completely liquefied and sub-cooled to the coldest temperature of the process, typically that of the stream of liquefied natural gas. At the coldest outlet of the exchanger, the refrigerant stream is expanded, forming a liquid phase and a gas phase. These two phases are remixed and reintroduced into

the exchanger. The refrigerant stream introduced in the two-phase state into the exchanger is vaporized there against the stream of hydrocarbons which liquefies. Documents WO-A-2017081374 and US2007-A-0227185 describe such known methods.

The use of aluminum brazed plate and fin heat exchangers results in very compact devices offering a large exchange surface area, which improves the energy performance of the liquefaction process described above.

These exchangers comprise a stack of plates which extend in two dimensions, length and width, thus constituting a stack of several series of passages, some being intended for the circulation of a circulating fluid, in this case the current of hydrocarbons to be liquefied, others being intended for the circulation of a refrigerant, in this case the two-phase refrigerant stream to be vaporized.

Heat exchange structures, such as heat exchange waves, are generally placed in the passages of the exchanger. These structures comprise fins which extend between the plates of the exchanger and make it possible to increase the heat exchange surface of the exchanger.

Conventionally, these heat exchange structures exhibit uniform properties and structures along the passages of the exchanger.

However, certain problems continue to arise with known liquefaction methods, in particular because of the two-phase composition of the refrigerant stream reintroduced into the exchanger and in particular when its vaporization takes place in an upward vertical flow.

Indeed, the two-phase refrigerant current is introduced at the cold end of the exchanger, that is to say the end where a fluid is introduced at the temperature is the lowest of the temperatures of the exchanger, located at the lower end of the exchanger. The partial vaporization rate (“flash” in English) is very low. As the refrigerant stream flows through the passages of the exchanger to the upper end forming the hot end, the rate of partial vaporization, and therefore the amount of gas contained in the refrigerant stream, increases.

However, the presence of gas is necessary to entrain the liquid phase of the refrigerant stream in order to compensate for the effect of gravity. As the quantity of gas is lower at the cold end of the exchanger, the entrainment of the liquid by the gas is more difficult there. The flow speed of the refrigerant stream is therefore lower at the cold end and then increases towards the upper end of the exchanger, as the refrigerant stream is vaporized. This results in an inhomogeneous distribution of the refrigerant stream along the length of the exchanger.

To remedy the shortage of gas at the cold end, a known solution is to reduce the section of the exchanger. The area available for the circulation of the refrigerant stream is reduced, which makes it possible to increase the volume flow and the flow rate of the refrigerant stream at the cold end.

However, this solution entails a major drawback. In fact, the section of the exchanger is sized considering the cold end, where the speed

refrigerant flow rate is the lowest. However, this speed continues to increase along the flow path of the refrigerant current, as the quantity of gas increases, which leads to a level of pressure drops that is much too great at the hot end, because of the reduced section of the exchanger.

This results in a degradation of the energy performance of the process. The object of the present invention is to resolve all or part of the above-mentioned problems, in particular by proposing a process for liquefying a stream of hydrocarbons against a two-phase refrigerant stream, said process using a heat exchanger ensuring a more homogeneous distribution of said refrigerant stream along the length of the exchanger.

The solution according to the invention is then a process for liquefying a stream of hydrocarbons such as natural gas using a heat exchanger comprising several plates parallel to each other and to a longitudinal direction which is substantially vertical, said exchanger having a length measured in the longitudinal direction, the plates being stacked with spacing so as to define between them at least a first series of passages for

the flow of at least part of a two-phase refrigerant stream vaporizing by heat exchange with at least the hydrocarbon stream, said process comprising the following steps:

a) introduction of the stream of hydrocarbons into the heat exchanger, b) introduction of a refrigerant stream into the heat exchanger through at least a first inlet to a first outlet, said first inlet and outlet being arranged so that the refrigerant stream flows through the exchanger in a downward direction opposite to the longitudinal direction,

c) outlet of the refrigerant stream through the first outlet of the exchanger,

d) expansion of the refrigerant stream from step c) so as to produce a two-phase refrigerant stream,

e) reintroduction of at least a part of the two-phase refrigerant stream into the heat exchanger through at least a second inlet to a second outlet, said second inlet and outlet being arranged so that said at least part of the refrigerant stream two-phase flows in the passages of the first series in an ascending direction oriented in the longitudinal direction, f) at least partially vaporization of said at least part of two-phase refrigerant current in the passages of the first series by heat exchange with at minus the stream of hydrocarbons so as to obtain a stream of at least partially liquefied hydrocarbons at the outlet of the exchanger, characterized in that at least one passage of the first series comprises a heat exchange structure comprising several series of fluid guide walls, said series of walls succeeding one another in the longitudinal direction and having leading edges which extend orthogonally to the longitudinal direction so as to face, in whole or in part, the two-phase refrigerant current, said heat exchange structure having a cross-sectional area of leading edges, measured orthogonally in the longitudinal direction and expressed per meter of heat exchanger length, decreasing in the longitudinal direction.

Depending on the case, the invention may include one or more of the following characteristics:

- in step e), the liquid / gas volume ratio of said at least part of the two-phase refrigerant stream reintroduced into the heat exchanger is between 10 and 100%, preferably between 10 and 60%, said at at least a part of the two-phase refrigerant stream flowing in the passages of the first series having a liquid / gas volume ratio which decreases in the longitudinal direction.

- the heat exchange structure is divided, along the longitudinal direction, into several portions each having a cross-sectional area of leading edges of predetermined value, a portion arranged downstream of another portion along the longitudinal direction, preferably consecutively, having a leading edge cross sectional area reduced relative to the leading edge cross sectional area of the other portion by a factor of at least 1.3, preferably a factor between 1, 5 and 5.

- the two-phase refrigerant current flowing through the portion arranged upstream of the other portion has a liquid / gas volume ratio at least 2% higher, preferably 2 to 20% lower, than the liquid / gas volume ratio two-phase refrigerant current flowing through the other portion.

the heat exchange structure is divided, along the longitudinal direction, into several portions each having a cross-sectional area of leading edges of predetermined value, said predetermined area values being decreasing in the longitudinal direction.

said portions form separate physical entities assembled together by brazing in said passage and / or at least one of said portions is formed of several distinct sub-portions assembled together by brazing in said passage.

said series of fluid guide walls each form a corrugation comprising a plurality of fins following one another in a lateral direction which is orthogonal to the longitudinal direction and parallel to the plates, with wave tops and wave bases alternately connecting said fins,

said corrugations have increasing pitches in the longitudinal direction, said pitches being defined as the distances between two successive fins of the same corrugation measured in the lateral direction.

the fluid guide walls have decreasing thicknesses in the longitudinal direction.

said series of fluid guide walls form corrugations having a corrugation direction oriented in the lateral direction, at least a portion of said corrugations exhibiting a predetermined offset in the lateral direction with respect to another adjacent corrugation, said offset corrugations having so-called tightening lengths, measured in the longitudinal direction, increasing in the longitudinal direction.

the heat exchange structure is divided, along the longitudinal direction, into several portions each comprising several series of fluid guide walls arranged consecutively in the longitudinal direction (z) with each series forming a corrugation, each portion (S1, S2) having corrugations having a predetermined offset in the lateral direction (y) with respect to another adjacent corrugation, each portion (S1, S2) comprising corrugations of a so-called clamping length (L1, L2) measured in the longitudinal direction ( z), the portions (S1, S2) being arranged in increasing order of their respective clamping length (L1, L2) along the longitudinal direction (z).

said portions (S1, S2) have at least one identical parameter of their undulations chosen from the predetermined offset, the thickness, the pitch.

in step a), the hydrocarbon stream is introduced in the gaseous or partially liquefied state into the heat exchanger at a temperature between -80 and -35 ° C.

in step a), the stream of hydrocarbons is introduced fully liquefied into the heat exchanger at a temperature between -130 to -100 ° C.

in step e), said at least part of the two-phase refrigerant stream is introduced into the heat exchanger at a first temperature of between - 120 and -160 ° C and leaves the heat exchanger at a second higher temperature at the first temperature, preferably the second temperature is between -35 and -130 ° C. in step e), the two-phase refrigerant stream introduced into the heat exchanger has a liquid / gas volume ratio of between 10 and 100%, preferably between 10 and 60%.

prior to step a), at least one cycle of

additional refrigeration comprising the following steps:

i) introduction of a feed stream comprising a mixture

hydrocarbons such as natural gas in an additional heat exchanger comprising a set of other plates parallel to each other and to the longitudinal direction and stacked with spacing so as to define between them at least one set of additional refrigerant passages,

ii) introduction of an additional refrigerant stream in the additional heat exchanger,

iii) extracting from the heat exchanger at least two refrigerant partial streams from the additional refrigerant stream and expansion of said refrigerant partial streams to different pressure levels to produce at least two two-phase refrigerants,

iv) reintroducing at least a portion of each refrigerant into respective additional refrigerant passages of the heat exchanger and at least partially vaporizing said at least a portion of each refrigerant by heat exchange with at least the stream of feed so as to obtain a pre-cooled stream of hydrocarbons at the outlet of the additional heat exchanger,

v) introduction of the pre-cooled hydrocarbon stream into the heat exchanger.

Preferably, the refrigerants flow upwardly in the longitudinal direction into the respective additional refrigerant passages of the heat exchanger.

More preferably, at least one additional refrigerant passage comprises at least one additional heat exchange structure comprising several additional series of fluid guide walls, said series succeeding each other in the longitudinal direction and having additional leading edges which overlap. extend orthogonally to the longitudinal direction so as to face, in whole or in part, the two-phase refrigerants, said exchange structure additional thermal having a cross-sectional area of leading edges, decreasing in the longitudinal direction.

The term "natural gas" refers to any composition containing

hydrocarbons including at least methane. This includes a "crude" composition (prior to any treatment or washing), as well as any composition that has been partially, substantially or fully treated for the reduction and / or elimination of one or more compounds, including, but not limited to. limit sulfur, carbon dioxide, water, mercury and certain heavy hydrocarbons and

aromatic.

The present invention will now be better understood thanks to the following description, given solely by way of nonlimiting example and made with reference to the appended figures, among which:

Fig. 1 schematically shows a process for liquefying an hydrocarbon stream according to one embodiment of the invention.

Fig. 2 is a schematic sectional view, in a plane parallel to the plates of the exchanger, of an exchanger passage configured for the flow of the two-phase refrigerant stream according to one embodiment of the invention.

Fig. 3 is a schematic sectional view, in a plane parallel to the plates of the exchanger, of an exchanger passage configured for the flow of the two-phase refrigerant stream according to another embodiment of the invention.

Fig. 4 shows a portion of the heat exchange structure of an exchanger according to one embodiment of the invention.

Fig. 5 shows a portion of the heat exchange structure of an exchanger according to another embodiment of the invention.

Fig. 6 shows a portion of the heat exchange structure of an exchanger according to another embodiment of the invention.

Fig. 7 shows a heat exchange structure of an exchanger according to another embodiment of the invention.

Fig. 8 shows schematically a process for liquefying an hydrocarbon stream according to another embodiment of the invention.

Fig. 1 shows schematically a process for liquefying a stream of hydrocarbons 102 which may be natural gas, optionally pre-treated, for example having undergone a separation of at least one of the following constituents: water, carbon dioxide, sulfur compounds, methanol , before its introduction into the heat exchanger E2. Preferably, the natural gas stream comprises, in mole fraction, at least 60% methane, preferably at least 80%.

The natural gas 102 can be fractionated, that is to say that a part of the C2 + hydrocarbons containing at least two carbon atoms is separated from the natural gas using a device known to those skilled in the art. The C2 + hydrocarbons collected are sent to fractionation columns comprising a deethanizer. The light fraction collected at the top of the deethanizer can be mixed with natural gas 102. The liquid fraction collected at the bottom of the deethanizer is sent to a depropanizer.

The hydrocarbon stream 102 and the refrigerant stream 202 enter

the exchanger E2 respectively by a third inlet 20 and a first inlet 21 to circulate therein through dedicated passages of the exchanger in directions parallel to the longitudinal direction z, which is substantially vertical in

operation. These currents come out through a third outlet 22 and a first outlet 23.

Advantageously, the first inlet 21 for the refrigerant stream 202 and the third inlet 20 for the hydrocarbon stream are arranged so that the refrigerant stream 202, and optionally the hydrocarbon stream 102, flow cocurrently in the descending direction, in the direction of a second end 2b of the exchanger which is located at a level lower than that of a first end 1a of said exchanger. Preferably, the first end 2a corresponds to the hot end of the exchanger E2, that is to say the entry point of the exchanger where a fluid is introduced at the highest temperature of the temperatures of the exchanger. , in this case the third entry 20.

Preferably, the hydrocarbon stream 102 is introduced in the fully gaseous or partially liquefied state into the heat exchanger E2 at a temperature between -80 and -35 ° C.

Alternatively, the stream of hydrocarbons 102 is introduced completely liquefied into exchanger E2 at a temperature between -130 and -100 ° C.

The refrigerant stream 201 leaving the exchanger E2 is expanded by an expansion member T3, such as a turbine, a valve or a combination of a turbine and a valve, so as to form a two-phase refrigerant stream 203 comprising a phase liquid and a gas phase. At least a part of the two-phase refrigerant stream 203 coming from the expansion is reintroduced into the exchanger E2 by at least a second inlet 41 located in the region of the second end 2b and supplying a first series of passages 10 of the exchanger.

Preferably, the second end 2b corresponds to the cold end, which corresponds to an entry point into the exchanger where a fluid is introduced at the lowest temperature of the exchanger temperatures, in this case the second inlet 41 Note that in the context of the invention, the reintroduction of said at least part of the two-phase refrigerant current 203 can be carried out in several ways. The two phases of the two-phase current 203 can be separated beforehand in a separator device 27 before being recombined outside the exchanger and reintroduced in the state of a liquid-gas mixture into the exchanger E2 via the same inlet 41 , as shown in Fig. 1. The separator member can be any device suitable for separating a two-phase fluid into a gas stream on the one hand and a liquid stream on the other hand. The two-phase current 203 is thus reintroduced entirely or almost entirely.

According to an alternative embodiment (not illustrated), the liquid and gaseous phases can be introduced separately into the exchanger by separate inlets, then mixed together within the exchanger, by means of a mixing device as described by example in FR-A-2563620 or WO-A-2018172644. These devices are typically machined parts comprising a particular arrangement of separate channels for a liquid phase and a gas phase and orifices putting these channels in fluid communication in order to distribute a liquid-gas mixture. The two-phase current 203 is thus reintroduced entirely or almost entirely.

According to another variant (not shown), only the liquid phase separated from the two-phase stream 203 is reintroduced through the second inlet 41. This liquid phase forms said part of the two-phase refrigerant stream 203. The gas phase is preferably diverted from the exchanger E2. , that is, it is not introduced there. Note that the two-phase fluid can optionally be reintroduced directly after expansion to the state of a liquid-gas mixture.

Preferably, said at least part of the two-phase refrigerant stream 203 is introduced into the heat exchanger E2 at a first temperature T1 of between -120 and -160 ° C and leaves the heat exchanger E2 at a second temperature T2 greater than the first temperature T1, preferably with T2 between -35 and -130 ° C. Said at least part of the two-phase refrigerant stream 203 flows through the passages 10 in an upward direction and is vaporized by countercurrently refrigerating the natural gas 102 and the refrigerant stream 202.

The vaporized refrigerant stream leaves the exchanger E2 via a second outlet 42 to be compressed by a compressor and then cooled in the indirect heat exchanger by heat exchange with an external cooling fluid, for example water or gas. 'air (at 26 in Fig. 1). The pressure of the refrigerant stream at the outlet of the compressor may be between 2 MPa and 8 MPa. The temperature of the refrigerant stream at the outlet of the indirect heat exchanger can be between 10 ° C and 45 ° C.

In the method described by FIG. 1, the refrigerant stream is not split into separate fractions, but, to optimize the approach in exchanger E2, the refrigerant stream can also be split into two or three fractions, each fraction being expanded to a different pressure level then sent to different stages of compressor K2.

Preferably, refrigerant stream 202 contains hydrocarbons having a carbon atom number of at most 5, preferably at most three, more preferably at most two.

Preferably, the refrigerant stream 202 is formed for example by a mixture of hydrocarbons and nitrogen such as a mixture of methane, ethane and nitrogen but can also contain propane, butane, pentane and / or ethylene.

The proportions in mole fractions (%) of the components of the refrigerant stream can be:

Nitrogen: 0% to 10%

Methane: 30% to 70%

Ethane: 30% to 70%

Propane: 0% to 10%

The natural gas leaves at least partially liquefied 220 from the exchanger E2 at a temperature preferably at least 10 ° C higher than the bubble temperature of the liquefied natural gas produced at atmospheric pressure (the bubble temperature designates the temperature at which the first vapor bubbles form in liquid natural gas at a given pressure) and at a pressure identical to the inlet pressure of natural gas, except for pressure drops. For example, natural gas leaves exchanger E2 at a temperature between -105 ° C and -145 ° C and at a pressure between 4 MPa and 7 MPa. In these conditions of

temperature and pressure, natural gas does not remain entirely liquid after expansion to atmospheric pressure.

Fig. 2 shows a passage 10 of an exchanger E2 according to the invention configured to vaporize the two-phase refrigerant stream. The exchanger E2 comprises several plates 2 (not visible) which extend in two dimensions, length Lz and width Ly of the exchanger, respectively in a longitudinal direction z and a lateral direction y orthogonal to z and parallel to the plates 2. The plates 2 are arranged parallel one above the other with spacing in a direction of stacking x, thus forming a plurality of passages for the fluids of the process which are to be put in indirect heat exchange relation via the plaques. A passage 10 is formed between two adjacent plates. Preferably, each passage of the exchanger has a parallelepipedal and flat shape. The gap between two successive plates is small compared to the length and width of each successive plate.

The hydrocarbon stream 102 circulates in a second series of passages (not shown) arranged, in whole or in part, alternating and / or adjacent to all or part of the passages 10 of the first series. The flow of fluids in the passages takes place generally parallel to the longitudinal direction z which is vertical during operation of the exchanger.

The sealing of the passages 10 along the edges of the plates is generally provided by lateral and longitudinal sealing strips 4 fixed to the plates. The lateral sealing strips 4 do not completely block the passages 10 but leave inlet 41 and outlet 42 openings. The inlets and outlets 41, 42 of the superposition of passages 10 are joined by collectors 71, 82 serving the introduction and discharge of the current 203.

Conventionally, the passages 10 include one or more heat exchange structures S arranged between the plates 2. The function of these structures is to increase the heat exchange surface of the exchanger. This is because the heat exchange structures are in contact with the fluids circulating in the passages and transfer heat flows by conduction to the adjacent plates.

The heat exchange structures also have a function of spacers between the plates 2, in particular during assembly by brazing the exchanger and to avoid any deformation of the plates during the use of the fluids under pressure. They also ensure the guidance of the fluid flows in the passages of the exchanger.

For convenience, it is usual to arrange heat exchange structures of the same type throughout an exchanger passage. For example, when these structures are formed from waves, they have corrugations of the same type, in particular the same period of ripple and therefore the same density of fins, same thickness, ...

However, the inventors of the present invention have demonstrated that with such a configuration, disparities in pressure drops and flow rates appear as the refrigerant current flows along the passages 10, due in particular to the progressive vaporization of said refrigerant stream.

In order to solve these problems, the invention proposes to arrange, in at least one passage 10 of the first series, a heat exchange structure making it possible to balance the pressure losses along the length of said passage.

More precisely, at least one passage 10 comprises a heat exchange structure S, the cross-sectional area of the leading edges of which decreases in the longitudinal direction z, that is to say in the direction of the first end of the exchanger.

Fig. 4 to Fig. 7 show examples of heat exchange structures that can be arranged in passage 10 and illustrate the leading edges of these structures. With reference to Fig. 4 and Fig. 5, a structure S according to the invention comprises several series of fluid guide walls 121, 122, 123 (a single series visible in Fig. 5), said walls being arranged parallel to the longitudinal direction z and having first edges d 'attack 124 arranged substantially

orthogonally to the longitudinal direction z and facing, in whole or in part, the two-phase refrigerant current when it flows in passage 10. The series of walls follow one another in the longitudinal direction z.

Fig. 4 shows walls 121, 122, 123 of thickness e1, measured in a plane orthogonal to the longitudinal direction z and in a direction orthogonal to the walls. The structure has a height h, measured in a stacking direction x which is orthogonal to the longitudinal direction z and orthogonal to the plates 2. According to the invention, the cross-sectional area of the structure is measured orthogonally to the longitudinal direction z and per meter of exchanger length. Determining the cross-sectional area A per unit length of exchanger makes it possible to qualify a progressive variation of said section along the passage 10 and / or to overcome any differences in length when considering different portions S1, S2, ... of structures (see below). For example, in Fig.5, the cross-sectional area A1 of the leading edges of the walls 121, 122, 123 corresponds to the hatched area.

The arrangement of an exchange structure whose leading edge cross-sectional area decreases in the direction of the first end 2a makes it possible to compensate for the disparities in pressure losses undergone by the refrigerant stream 203 along the passage 10.

Thus, at the second end 2b, where the quantity of gas present in the current is still relatively low, a larger leading edge surface area per unit length makes it possible to increase the pressure drops and the speed

flow, promoting the ascent of the current 203. As it flows in the longitudinal z direction, reducing the surface of the leading edges makes it possible to reduce the pressure losses undergone by the refrigerant current 203.

The exchanger according to the invention makes it possible to adjust the pressure drops along the length of the passage and to maintain a reasonable level of pressure drops at the first end 2a. The energy performance of the industrial installation incorporating the exchanger according to the invention is improved.

It also allows for sufficiently high fluid flow velocities along the entire length of the passage, especially at the second end 2b where the entrainment of the liquid phase is critical. This results in a more uniform distribution of the two-phase refrigerant flow and an improvement in the performance of the exchanger. The exchanger can thus be dimensioned with reduced safety margins compared to the margins that should be provided in the absence of structures according to the invention.

In addition, the exchanger can operate in so-called reduced steps, that is to say lower in flow, whether in transient operating mode or in steady state.

According to an embodiment shown schematically in FIG. 2, the exchange structure S has a section area of leading edges A which gradually decreases, monotonously or not, in the z direction. The structure S can be formed in one piece, ie from the same plate, or else from different distinct parts juxtaposed in the z direction.

According to another embodiment shown schematically in FIG. 3, the heat exchange structure S is divided, along the longitudinal direction z, into several portions S1, S2, ... each having a cross-sectional area A1, A2 ... with leading edges of predetermined value, said predetermined surface values being decreasing in the longitudinal direction z.

More precisely, said portions each have a constant leading edge area, the reduction of said area being obtained by a variation from one portion to another.

Fig. 3 shows schematically a structure S with three portions S1, S2, S3, it being specified that the structure S comprises at least two portions and can comprise a greater number of portions. The description below for two servings is

transposable to a greater number of portions.

Preferably, said portions S1, S2, ... form distinct physical entities, formed from distinct strips. For example the portions S1, S2 are separate wave mats. Advantageously, the structural portions are assembled together by brazing in the passage 10. One and / or the other of said portions S1, S2 can also be formed of several distinct sub-portions, preferably in the form of a carpet. waves, assembled together by brazing in said passage 10. Preferably, a portion S2 arranged upstream of another portion S1 along the longitudinal direction z, preferably consecutively, has a cross-sectional area A2 of leading edges increased by a multiplying factor of at least 1, 3, preferably between 1, 5 and 5 with respect to the cross-sectional area A1 of the leading edges of the other portion S1.

Such a multiplying coefficient makes it possible to effectively balance the pressure losses undergone by the two-phase refrigerant current, in particular when said current flows through the other portion S1 with a lower liquid / gas volume ratio of at least 2%, preferably 2 to 20% less than the liquid / gas volume ratio of the stream flowing in the portion S2, which is closer to the second end 2b. In this case, it is a question of average liquid / gas volume ratios over the lengths of each portion considered. Note that, preferably, the two-phase refrigerant stream introduced into the heat exchanger E2 has a liquid / gas volume ratio of between 10 and 100%, preferably between 10 and 60%, said ratio being defined as the ratio between the liquid phase volume flow rate and the gas phase volume flow rate of the two-phase refrigerant stream.

Advantageously, each series of fluid guide walls 121, 122, 123, 221, 222, 223 forms a corrugation comprising a plurality of fins 123, 223 succeeding one another in the lateral direction y, with wave tops 121, 221 and wave bases 122, 222 alternately connecting said fins 123, 223. The fins preferably follow one another periodically. Said corrugations have pitches p1, p2 defined as the distances between two successive fins of the same corrugation measured in the lateral direction y. To express the pitches p1 and p2 of the corrugations, we can use the relations p1 = 25.4 / n1 and p2 = 25.4 / n2 with n1 and n2 respectively representing the number of fins 123, 223 per inch, 1 inch being equal to 25.4 millimeters, corrugations, measured in the lateral x direction.

Preferably, the first and second fluid guide walls extend parallel to the longitudinal direction z. They can also be arranged parallel or orthogonally to the plates 2.

Preferably, said corrugations have increasing pitches in the longitudinal direction z. In other words, the corrugations have a decreasing fin density in the longitudinal direction z.

Alternatively or in addition, said corrugations have wall thicknesses which decrease in the longitudinal direction z.

Increasing the thickness or reducing the pitch of the fins makes it possible to increase the transverse surface area of the leading edges seen by the two-phase refrigerant flow towards the second end of the exchanger, which tends to increase the pressure losses of load, and therefore the flow velocity in this zone.

For example, if we consider a structure S comprising a first and a second portion S1, S2, the first portion S1 being arranged downstream of the second portion S2, that is to say further from the second end 2b , the walls of the second portion 221, 222, 223 have a second thickness e2 greater than the first thickness e1 of the walls 121, 122, 123 of the first portion S1. The term "downstream" is used considering the direction of flow of the two-phase current 203 in the portions S1, S2 ... The first portion S1 may have a first ripple pitch p1 greater than the second ripple pitch p2 of the second portion S2.

As corrugations for the series of walls of the heat exchange structure S, we can use the different types of waves usually implemented in exchangers of the brazed plate and fin type. The waves can be chosen from known wave types such as straight waves, so-called partially offset waves (of the "serrated" type), wave waves or herringbones (of the "herringbone" type in English). . These waves may or may not be perforated.

Fig. 5 shows an embodiment in which a series of fluid guide walls 121, 122, 123, which may constitute a portion S1 of the structure, forms a corrugation of the right wave type. This series of walls has a cross-sectional area A1 of leading edges 124.

Fig. 6 and Fig. 7 show embodiments in which several series of fluid guide walls, which may constitute several portions of structure S1, S2, form partially offset waves.

As seen in Fig. 7, the portion S2 comprises several series of fluid guide walls 221 i, 222i, 223i, 221 i + 1, 222i + 1, 223i + 1, 221 i + 2, 222i + 2, 223i + 2. The series follow one another in the longitudinal direction z and each form an undulation having a direction of undulation parallel to the lateral direction y. Each corrugation is offset by a predetermined distance d2, in the lateral direction y, with respect to an adjacent corrugation. The corrugations have a so-called tightening length L2 measured in the longitudinal direction z.

In the case of a partially offset wave, the cross-sectional area A2 of the leading edges of the portion S2 corresponds to the sum of the cross-sectional areas A2i, A2i + 1, A2i + 2, measured orthogonally to the direction

longitudinal z and expressed per meter of heat exchanger length, leading edges 224i, 224i + 1, 224i + 2 of each series of guide walls.

The above description can be transposed to the portion S1 shown in FIG. 6.

In the context of the invention, the variation of the cross section of the leading edges of the exchange structure S along the longitudinal direction z may be obtained by variation of at least one characteristic dimension, such as thickness, pitch. wave, tightening length ... within the structure. In particular, this variation could take place between portions of structures of the same type. For example, the heat exchange structure S may include several partially offset wave portions, the clamping length increasing towards the first end.

Fig. 6 and Fig. 7 illustrate such an embodiment in which the heat exchange structure S comprises at least two portions S1, S2 in the form of partially offset waves. Advantageously, the portion S2 arranged upstream of the other portion S1, has a clamping length L2 is less than that L1 of the other portion S1. This makes it possible to arrange more leading edges per meter of heat exchanger length on the side of the second end 2b, and therefore to increase the transverse area of the leading edges and the pressure losses which result therefrom on the fluid flowing in front of these leading edges.

Preferably, a clamping length L1 will be chosen for the other portion S1 greater than the clamping length L2 of the portion S2 by a factor of between 1, 7 and 7, this for a portion S2 arranged upstream to another portion S1, preferably adjacent. The clamping lengths may be between 1 and 15 mm, preferably between 3 and 13 mm.

Preferably, the offset distances are between 1 and 20 mm, preferably between 3 and 15 mm.

Preferably, the characteristic dimensions of the waves other than the clamping lengths, such as offset distances, thickness, not corrugations ... are identical within the S exchange structure.

A variation of the wave type can also be implemented within the structure S to balance the pressure drops undergone by the refrigerants on these two portions. For example, arranging one or more portions S2, S3 partially offset on the side of the second end and arranging one or more portions S1, S2 in a right wave form on the side of the first end. A straight wave indeed introduces fewer leading edges into the passage and therefore less pressure drop.

With reference to Fig. 4, Fig. 5, Fig. 6 or Fig. 7, note that for a given heat exchange structure S1 or S2 comprising fluid guide walls of thickness e1 or e2 forming at least a first corrugation of pitch p1 or p2, of height h1 or h2, it is possible to define the cross-sectional areas A1, A2 per meter of exchanger length using the following relationships: Math 1

with Ly the width of passage 10 in which the two-phase refirgant current flows and

K1 or K2 equal to 1 in the case where portion S1 or S2 is a straight wave, that is to say whose fluid guide walls form a single undulation, without offset,

or

K1 = 1000 / L1 or K2 = 1000 / L2 in the case where the portion S1 or S2 is a partially offset wave with several offset corrugations, with L1 or L2 the clamping lengths expressed in millimeters for S1 or S2.

For example, for a partially offset wave S2 called “1/8” serrated ”(1” = 1 inch = 25.4 mm), we have L2 = 25.4 / 8 = 3.18 mm. For a partial shift wave S1 called “1/5” serrated ”(1” = 1 inch = 25.4 mm), we have L1 = 25.4 / 5 = 5.08 mm.

Advantageously, the process for liquefying an hydrocarbon stream according to the invention can implement one or more refrigeration cycles

carried out upstream of the main refrigeration cycle described above, so as to pre-cool the stream

of hydrocarbons.

Fig. 8 shows schematically a process for liquefying a stream of hydrocarbons such as natural gas comprising an additional refrigeration cycle in which the natural gas is cooled to its dew point using at least two levels of expansion different to increase cycle efficiency. This additional refrigeration cycle is operated by means of an additional refrigerant stream in an additional heat exchanger E1, called the pre-cooling exchanger, arranged upstream of the heat exchanger E2, which then forms the liquefaction exchanger.

In this embodiment, the feed stream 1 10 arrives for example at a pressure of between 4 MPa and 7 MPa and at a temperature of between 30 ° C. and 60 ° C. The feed stream 110 comprising a mixture of hydrocarbons such as natural gas, the refrigerant stream 202, the additional refrigerant stream 30 enter the exchanger E1 to circulate therein in parallel directions and at co-current in the downward direction. .

A cooled stream of hydrocarbons 102, or even at least partially liquefied, leaves the pre-cooling exchanger E1. Preferably, the hydrocarbon stream 102 exits in the gaseous or partially liquefied state, for example at a temperature between -35 ° C and -70 ° C. Preferably, the refrigerant stream 202 leaves completely condensed from the exchanger E1, for example at a temperature between - 35 ° C and - 70 ° C. The stream 102 is then introduced into the exchanger E2. As seen in Fig. 8, the vaporized refrigerant stream leaves the exchanger E2 to be compressed by the compressor K2 and then cooled in the indirect heat exchanger C2 by heat exchange with an external cooling fluid, for example water or water. air. The second refrigerant stream from exchanger C2 is sent to exchanger E1 via line 20.

The additional refrigerant stream can be formed by mixing

hydrocarbons such as a mixture of ethane and propane, but may also contain methane, butane and / or pentane. The proportions in molar fraction (%) of the components of the first cooling mixture can be:

Ethane: 30% to 70%

Propane: 30% to 70%

Butane: 0% to 20%

In the method described by FIG. 8, the refrigerant stream 202 is not split into separate fractions, but, to optimize the approach in exchanger E2, the refrigerant stream can also be split into two or three fractions, each fraction being expanded to a pressure level different then sent to different stages of compressor K2.

In the additional exchanger E1, which is also of the plate and fin type, at least two partial streams from the additional refrigerant stream are withdrawn from the exchanger at two separate outlet points and then expanded to different pressure levels, thus forming at least a first and a second distinct refrigerant fluids F1 and F2 reintroduced into the exchanger through separate inlets 31, 32 selectively supplying refrigerant passages to be vaporized therein with the feed stream, the refrigerant stream and part of the additional refrigerant stream.

In the embodiment according to FIG. 8, three fractions, also called partial flows or streams, 301, 302, 303 of the additional refrigerant stream 30 in the liquid phase are successively withdrawn. The fractions are expanded through the expansion members, such as valves, turbines or combinations of valves and turbines, V11, V12 and V13 at three different pressure levels, forming a refrigerant F1, a second refrigerant F2 and a third refrigerant F3. These three refrigerants F1, F2, F3 are reintroduced into the additional exchanger E1 and then vaporized.

The three vaporized refrigerants F1, F2, F3 are sent to different stages of compressor K1, compressed and then condensed in condenser C1 by heat exchange with an external cooling fluid, for example water or air. The first refrigerant stream issuing from the condenser C1 is sent to the additional exchanger E1 via the pipe 30. The pressure of the first refrigerant stream at the outlet of the compressor K1 can be between 2 MPa and 6 MPa. The temperature of the additional refrigerant stream at the outlet of the condenser C1 can be between 10 ° C and 45 ° C.

Preferably, the refrigerants F1, F2, F3 flow from one end 1b of the additional exchanger E1 to another end 1a in the longitudinal direction z, in the upward direction. End 1b corresponds to the cold end of the additional exchanger E1 where the refrigerant F1 is introduced to the

lowest temperature of the additional heat exchanger E1. Advantageously, the exchanger E1 comprises at least one of the passages for the flow of refrigerants from the pre-cooling cycle in which at least one additional heat exchange structure is arranged having a cross-sectional area of leading edges which decreases in the z direction. Said additional exchange structure may have one or more of the characteristics described above.

The arrangement of exchange structures having different leading edge cross-sectional areas makes it possible to balance the pressure drops undergone by the refrigerants along the refrigerant passages, while keeping a reasonable level of pressure drops at the temperature. 'other end 1 a of the exchanger E1. The The energy performance of the industrial installation incorporating the E1 exchanger is further improved.

Of course, the invention is not limited to the specific examples described and illustrated in the present application. Other variants or embodiments within the reach of those skilled in the art can also be envisaged without departing from the scope of the invention. For example other configurations of injection and extraction of fluids from the exchanger, other direction and directions of flow of fluids, other types of fluids, other types of heat exchange structures ... are of course possible, depending on the constraints imposed by the process to be implemented.

Claims

1. A method of liquefying a stream of hydrocarbons (102) such as natural gas using a heat exchanger (E2) comprising several plates (2) parallel to each other and to a longitudinal direction (z) which is substantially vertical, said exchanger (E2) having a length measured in the longitudinal direction (z), the plates (2) being stacked with spacing so as to define between them at least a first series of passages (10) for the flow of at least part of a two-phase refrigerant stream (203) vaporizing by heat exchange with at least the hydrocarbon stream (102), said method comprising the following steps:

a) introduction of the stream of hydrocarbons (102) into the heat exchanger (E2),

b) introduction of a refrigerant stream (202) into the heat exchanger (E2) through at least a first inlet (21) up to a first outlet (23), said first inlet and outlet (21, 22) being arranged so that the refrigerant flow

(202) flows into the exchanger (E2) in a downward direction opposite to the longitudinal direction (z),

c) outlet of the refrigerant stream (201) through the first outlet (22) of the exchanger (E2),

d) expansion of the refrigerant stream (201) from step c) so as to produce a two-phase refrigerant stream (203),

e) reintroduction of at least a part of the two-phase refrigerant stream (203) in the heat exchanger (E2) through at least a second inlet (41) to a second outlet (42), said second inlet and outlet ( 41, 42) being arranged so that said at least part of the two-phase refrigerant stream

(203) flows through the passages (10) of the first series in an upward direction oriented in the longitudinal direction (z),

f) at least partially vaporizing said at least part of two-phase refrigerant stream (203) in the passages (10) of the first series by heat exchange with at least the hydrocarbon stream (102) so as to obtain a stream of at least partially liquefied hydrocarbons (220) at the outlet of the exchanger (E2), characterized in that at least one passage (10) of the first series comprises a heat exchange structure (S) comprising several series of fluid guide walls (121, 122, 123, 221, 222, 223), said series of walls following one another in the longitudinal direction (z) and having leading edges (124, 224) which extend orthogonally to the longitudinal direction (z) so as to face, in whole or in part, the two-phase refrigerant current (203), said heat exchange structure (S) having a cross-sectional area (A) of leading edges, measured orthogonally to the longitudinal direction (z) and expressed per meter of heat exchanger length, decreasing in the longitudinal direction (z).

2. Method according to claim 1, characterized in that in step e), the liquid / gas volume ratio of said at least part of the two-phase refrigerant stream (203) reintroduced into the heat exchanger (E2) is between 10 and 100%, preferably between 10 and 60%, said at least part of the two-phase refrigerant current (203) flowing in the passages (10) of the first series having a following decreasing liquid / gas volume ratio The direction

longitudinal (z).

3. Method according to one of claims 1 or 2, characterized in that the heat exchange structure (S) is divided, along the longitudinal direction (z), into several portions (S1, S2, ...) having each a cross-sectional area (A1, A2 ...) of leading edges of predetermined value, a portion (S1) arranged downstream of another portion (S2), preferably consecutively, following the longitudinal direction ( z) having a cross-sectional area (A1) of leading edges reduced compared to the cross-sectional area (A2) of leading edges of the other portion (S2) by a factor of at least 1 , 3, preferably a factor between 1, 5 and 5.

4. Method according to claim 3, characterized in that the two-phase refrigerant current (203) flowing through the portion (S2) arranged upstream of the other portion (S1) has a higher liquid / gas volume ratio of at least 2%, preferably 2 to 20% less, than the liquid / gas volume ratio of the two-phase refrigerant stream (203) flowing through the other portion (S1).

5. Method according to one of claims 3 or 4, characterized in that said portions (S1, S2, ...) form distinct physical entities assembled together by brazing in said passage (10) and / or at least l one of said portions (S1, S2) is formed of several distinct sub-portions assembled together by brazing in said passage (10).

6. Method according to one of the preceding claims, characterized in that said series of fluid guide walls (121, 122, 123, 221, 222, 223) each form a corrugation comprising a plurality of fins (123, 223). ) succeeding each other in a lateral direction (y) which is orthogonal to the longitudinal direction (z) and parallel to the plates (2), with wave tops (121, 221, 321) and wave bases (122, 222) alternately connecting said fins (123, 223).

7. Method according to claim 6, characterized in that said corrugations have increasing pitches in the longitudinal direction (z), said pitches being defined as the distances between two successive fins of the same corrugation measured in the lateral direction (y). , and / or the fluid guide walls (121, 122, 123, 221, 222, 223) forming said corrugations have decreasing thicknesses in the longitudinal direction (z).

8. Method according to one of claims 6 or 7, characterized in that said series of fluid guide walls (121, 122, 123, 221, 222, 223) form corrugations each having a direction of corrugation oriented in parallel. in the lateral direction (y), at least part of said corrugations having a predetermined offset in the lateral direction (y) with respect to another adjacent corrugation, said offset corrugations having so-called clamping lengths, measured in the longitudinal direction ( z), increasing in the longitudinal direction (z).

9. Method according to one of claims 6 to 8, characterized in that the heat exchange structure (S) is divided, along the longitudinal direction (z), into several portions (S1, S2, ...) comprising each several series of fluid guide walls arranged consecutively in the longitudinal direction (z) with each series forming a corrugation, each portion (S1, S2) having corrugations having a predetermined offset in the lateral direction (y) with respect to another adjacent corrugation, each portion (S1, S2) comprising corrugations of a so-called clamping length (L1, L2) measured in the longitudinal direction (z) , the portions (S1, S2) being arranged in increasing order of their respective clamping length (L1, L2) along the longitudinal direction (z).

10. The method of claim 9, characterized in that said portions (S1, S2) has at least one identical parameter of their undulations chosen from the predetermined offset, the thickness, the pitch.

11. Method according to one of the preceding claims, characterized in that in step a), the hydrocarbon stream (102) is introduced in the gaseous state or

partially liquefied in the heat exchanger (E2) at a temperature between -80 and -35 ° C.

12. Method according to one of the preceding claims, characterized in that in step e), said at least part of the two-phase refrigerant stream (203) is reintroduced into the heat exchanger (E2) at a first temperature. (T1) between -120 and -160 ° C and leaves the heat exchanger (E2) at a second temperature (T2) higher than the first temperature (T1), preferably with T2 between -35 and -130 ° C.

13. Method according to one of the preceding claims, characterized in that, prior to step a), at least one additional refrigeration cycle is implemented comprising the following steps:

i) introduction of a feed stream (110) comprising a mixture of hydrocarbons such as natural gas in a heat exchanger (E1)

additional comprising a set of other plates parallel to each other and to the longitudinal direction (z) and stacked with spacing so as to define between them at least one set of additional refrigerant passages,

ii) introduction of an additional refrigerant stream (30) in the additional heat exchanger (E1),

iii) extraction from the heat exchanger (E1) of at least two refrigerant partial streams (301, 302) from the additional refrigerant stream (30) and expansion said partial refrigerant streams (301, 302) at different pressure levels to produce at least two two-phase refrigerants (F1, F2),

iv) reintroduction of at least a portion of each refrigerant (F1, F2) into respective additional refrigerant passages of the heat exchanger (E1) and at least partially vaporization of said at least a portion of each refrigerant (F 1 , F2) by heat exchange with at least the feed stream (110) so as to obtain a pre-cooled hydrocarbon stream (102) at the outlet of the additional heat exchanger (E1),

v) introduction of the pre-cooled hydrocarbon stream (102) into the heat exchanger (E2).

14. The method of claim 13, characterized in that the refrigerants (F1, F2) flow ascending in the direction

longitudinal (z) in the respective additional refrigerant passages of the heat exchanger (E1).

15. Method according to one of claims 13 or 14, characterized in that at least one additional refrigerant passage comprises at least one additional heat exchange structure comprising several additional series of fluid guide walls, said series following one another according to the longitudinal direction (z) and having additional leading edges which extend orthogonally to the longitudinal direction (z) so as to face, in whole or in part, the two-phase refrigerants (F1, F2), said structure d additional heat exchange having a cross-sectional area of leading edges, decreasing in the longitudinal direction (z).